Application Potentials of Sensor Network and Ad Hoc Network

1 The Status Quo of Wireless Sensor Network (WSN) and Ad Hoc Network

1.1 The Status Quo of WSN
The combination of Computing, Communication and Sensor, three cornerstones of the information industry of the 21st century, has given the birth of WSN. The leading nations in science and technology have paid great attention to WSN, and made relevant national policies and development strategies.

    The United States makes a point of the research into WSN theory and its military and civil applications. US National Science Foundation (NSF) has made the study plan of WSN, and funded the study on its basic theories. Based on the C4ISR plan, US Department of Defense launched C4KISR, in which WSN is an important study subject. Moreover, a number of universities, research institutes and corporations in US have initiated their studies on WSN. Crossbow, Dust Networks(tm), Ember, Chips, Freescale, Intel Corporations have launched WSN chips, node equipment and solutions for commercial use.

    The Sixth EU Framework Program prioritized the development of  "Information Society Technologies", which involves the study of WSN. Besides, Philips, Siemens, Ericsson, France Telecom, Chipcon and ZMD have also initiated their R&D of WSN, launched relevant products, and provided solutions for diversified applications.
The Ministry of Internal Affairs and Communications of Japan set up a study group on ubiquitous sensor network, with the purpose of studying and discussing its relevant R&D programs, standardization programs, cognition of the society and policy promotion. Some enterprises such as NEC and OKI have launched WSN products and made certain application trials.

    The Ministry of Information and Communication of South Korea has made the IT 839 Strategy, out of which "3" refers to the infrastructure of the IT industry: Broadband Convergence Network (BCN), ubiquitous sensor network

    and next-generation IP. South Korea has taken specific actions to push the implementation of the IT839.
The development of the sensor industry has been prioritized in both the perspective long-term plan and the Eleventh Five-Year Plan of China. Meanwhile, the China Next Generation Internet (CNGI) Program and the National Natural Science Foundation support the study of WSN well. Presently, a number of research institutes, universities and enterprises in China have initiated their relevant research projects. [1—3]

1.2 The Status Quo of Ad Hoc Network
Ad hoc Network is a temporary, multihop and peer-to-peer self-organizing system composed of mobile nodes. The earliest study on Ad hoc network started at Packet Radio Network (PRNET) in Battle Environments, funded by the Defense Advanced Research Projects Agency (DARPA) in 1970s. Subsequently, the DARPA funded Survivable Adaptive Networks (SURAN) and Global Mobile Information Systems (GloMo) programs respectively in 1983 and 1994. The IEEE 802.11 standards committee set up in 1991 first used "Ad hoc network" to describe such special peer-to-peer wireless mobile network. Currently, the main study subjects include the Ad hoc network, the Ad hoc model in broadband, wireless access system and multihop based enhancement technology for public cellular mobile communication systems.

1.3 Differences Between WSN and Ad Hoc Network
WSN is a typical application of the Ad hoc network. However, WSN differs from conventional Ad hoc networks in the following aspects. Firstly, the number of WSN nodes can be several orders of magnitude higher than that of nodes is an ad hoc network, and sensor nodes are densely deployed. Secondly, sensor nodes are prone to failures, and the topology of WSN changes very frequently. Thirdly, WSN mainly uses a broadcast communication paradigm, while the Ad hoc network is based on point-to-point communications. Fourthly, WSN is limited in power, computational capacities and memory. Fifthly, sensor nodes may not have global identification. Last, WSN focuses on data. There are obvious differences between the two networks. Therefore, numbers of research achievements in the Ad hoc network cannot be applied to WSN. Besides, these differences lead to greatly different applications of the two networks.

2 Analysis of Market Demand for WSN and Ad Hoc Network
Essentially, three important factors push the development of WSN.

    (1) The Wireless Effects
    Demands for wireless connectivity is evident across many markets. The trend to go wireless extends beyond home, campus and office applications. The spectrum is wireless technology applied to sensing, monitoring and control applications. WSN is applicable to distributed networks with low data rates, and can be applied in such fields as remote automatic monitoring and control.

    (2) Economic Driving Forces
    Reducing operational and labor costs and improving efficiency are economic driving forces of WSN. Moreover, WSN may bring good social benefits. For example, it helps save the power consumption of electronic devices and appliances in office and relieve traffic congestion.

    (3) Technology Driving Forces
    Technology advance keeps reducing the cost of hardware. The development of the WSN market will benefit from low-price Radio Frequency (RF) modules.

    Micro-Electro-Mechanical System (MEMS), nano-technologies and multifunctional sensor technologies help produce micro sensor equipment with lower cost and capability of more precise sensing capabilities. Moreover, improved power characteristics will eventually allow WSN to operate continuously, enabling new applications and new markets.

    The market of the Ad hoc network is also pushed by the above-mentioned factors. The Ad hoc network is independent of network infrastructure. It features simple deployment, rapid expansion and good invulnerability. Therefore, it can meet numerous special demands from various application fields. The Ad hoc network is generally applicable to the following situations:

• The network equipment is unable or inconvenient to be installed in advance;
• It is necessary to quickly self-organize the network;
• The communication equipment is destroyed;
• Distributed network is required.
  

    Besides, it can be regarded as a backup network with survival ability.

3 Typical Applications of WSN  and Ad Hoc Network

3.1 Typical Applications of WSN
Sensor networks make it possible to sense the situations of human and objects, as well as environments, and to offer diversified services to the users with different demands. Peoples are not satisfied with information transmission and searches anymore. They also want to acquire information. WSN can meet this emerging demand. Moreover, Machine-to-Machine (M2M) communication will dominate the future communication network, and WSN can satisfy this new communication model extensively. Japanese study group on the ubiquitous sensor network divides the applications of WSN into 14 kinds that belong to three categories: "Comfort", "Safety" and "Efficiency". WSN applications are summed up here, as shown in Figure 1.

    (1) Logistics
    Radio Frequency Identification (RFID) is an important part of WSN. When applied into logistics, stock and assets management, RFID or sensor-enhanced RFID plays an important role in quickening commodity circulation and guaranteeing commodity quality while helps retailers make good profits. With RFID, the entire process from commodity production to sale can be followed. This helps not only effectively avoid the loss of goods, but also acquire information about the sales period to prevent shortage.

    (2) Intelligent Transportation
    WSN has an enormous application potential in intelligent transportation. Sensor nodes are installed at the sides of roads to establish a WSN to acquire transportation information. Based on it, intelligent transportation can be implemented, including transportation control, transportation guidance, priority of emergency vehicles, provision of information about parking lots, toll payment without stopping, and accident avoidance.

    (3) Healthcare
    Combined to home nursing and remote health care WSN will greatly improve the level of healthcare. Main applications include remote health management, ICU patients and the aged care, life supporting devices, real-time collection and management of pathological data, and emergency ambulance. Some institutes such as University of Rochester and Intel are making relevant experiments. TeliaSonera, an operator in North Europe and Baltic States, is going to launch BodyKom(tm), a service for remote healthcare.

    (4) Industrial Monitoring and Control
    In the industry, WSN is mainly applied for monitoring and control of large-size equipment to obtain its running situation and working environments. With real-time supervision of the working status of equipment, some serious hidden safety issues can be avoided.

    (5) Security 
    In the field of security, WSN is mainly applied into prevention from unwarranted invasion, check for guard against theft, check for dangerous objects, and industrial security.

    (6) Infrastructure Control
    WSN helps implement the online monitoring on infrastructure such as bridge, building, aqueduct, electricity and gas. With it, the situation of important infrastructure can be known in real time. Based on the real-time monitoring of the sensor on damage and degradation of infrastructure, the protecting plan may keep optimizing, and the life span of the infrastructure is accordingly prolonged. In case that a disaster happens, the sensor could be used to diagnose the security status of the infrastructure to ensure it is safe for use.

    (7) Military and National Defense
Presently, military is the most important application field of WSN. Since it requires no equipment installation, supports quick expansion, and has strong destruction immunity, WSN has become an important technique for intelligence gathering in the territory of hostile forces. Its applications in military include identification and tracing of forces, accoutrements and provisions, and monitoring of the status in conflict zones. Additional applications are spying on the terrain and fortifications distribution of enemy; location of attack objects; evaluation of loss at the battlefield; spying and detecting nuclear, biological and chemical attacks.

    (8) Ecological Environments
    WSN facilitates data gathering, and avoids possible damages to the environment caused by human´s invasion for traditional data gathering. The important applications of WSN in biological environments include environment monitoring, monitoring of the Earth, waste tracing, and optimization of energy demands. Several application trials have been done. For example, NEC developed a special multihop sensor network for environment monitoring. Moreover, Intel put the iMote in the nest of seabird to monitor temperature, wind power and activities of the bird.

    (9) Disaster Prevention and Relief
    Sensors are deployed in human´s unreachable places or a large-scale area to automatically establish a WSN. It automatically detects earthquake, flood and severe tropical storm, and releases the evacuation order and warning. Besides, if the disaster destroys the original infrastructure, it is able to fulfill certain communication search and rescue tasks.

    (10) Agriculture and Food
    WSN is generally applied in the real-time monitoring of the environment for growth of the crops, which helps optimize the environment. For example, it is used in the field to gather information about composition of soil, sunshine and humidity. It is also used to analyze abnormal phenomena, based on which plant diseases and insect pests are cured, and the pesticide is sprayed at the best time.

3.2 Typical Applications of Ad hoc Network
The Ad hoc network features peer-to-peer nodes, no central access for multihop, independence of network infrastructure and excellent destruction immunity. Therefore, its applications are greatly different from those of ordinary communication networks. So far, military is still a main application field of the Ad hoc network. With regard to the application in the civil field, its characteristics of self-organization and multihop, makes it possible to gradually integrate the existing mobile communication systems and Ad hoc network, which enables the Ad hoc network to have wide applications, as shown in Figure 2.

    (1) Battlefield Communication 
    Military is the earliest and most important application field of the Ad hoc network; conversely, the Ad hoc network is regarded as the optimum technology for communication at digital battlefield. Currently, Ad hoc has been the core technology for US Army´s tactical Internet. Major equipment of US Army´s digital radio and wireless Internet controller uses the mobile Ad hoc technology. For example, US DARPA funded the Self-healing Minefield Program. In a self-healing minefield, all mines can quickly establish a mobile Ad hoc network to close any breach immediately.

    (2) Emergency and Temporary Communication
    After an earthquake, flood, severe tropical storm, tsunami or other disasters, the fixed communication infrastructure is possibly destroyed or out of work. At this time, the Ad hoc network, independent of any fixed network infrastructure and deployed quickly, can quickly establish a mobile communication network. Similarly, it is also the best choice for communication in remote areas lacking fixed network infrastructure. Moreover, it is applicable in places with demand of temporary communication, such as exhibition centers, meeting rooms, the gymnasium, classrooms and train.

    (3) Sensor Network
    A large amount of field measurement and data gathering requires wireless communication technologies, and WSN is a good choice with obvious strengths. Scattered in a dangerous environment, the sensors organize an Ad hoc network to implement communication between sensors and the control center. Therefore, it is possible for relief staff out of the locale to gather and distinguish the accident information sent by the sensors in the dangerous locale.

    (4) Wireless Personal Area Network (WPAN)
    WPAN is established for wireless communication in a short distance between personal electronic communication equipment, such as the Personal Digital Assistant (PDA), mobile phone and laptop computer. It has the characteristics of dynamic networking, and belongs to the Ad hoc network. Neither base stations nor the mobile switch center is required in WPAN. Once two mobile nodes are close enough, they can directly set up connection by the wireless channel. Therefore, it implements wireless dynamic connection and real-time information exchange between these devices.

    (5) Mesh Wireless Local Area Network (Mesh WLAN)
    The mesh WLAN is one of the main application fields of the Ad hoc network. The IEEE 802.11 has two models: the basic-network model and the Ad hoc model. In the former, user equipment communicates with the access point that is connected to the basic network. However, the access point is saved in the latter model, and user devices directly communicate with each other. The combination of Ad hoc and WLAN produces the mesh WiFi network, which expands the coverage of WLAN by multihop technology.

    (6) Multihop Based Enhancement of Cellular Mobile Communication Systems
The Ad hoc technology can be integrated with the cellular mobile communication system. The capability of multihop forwarding of the mobile terminal is used to expand the coverage of the cellular mobile communication system, to balance traffic of adjacent cells, and to improve the data rate at the cell edge. Therefore, the Ad hoc is expected to become an important part of the 4G network. Presently, there are numbers of research projects on Ad hoc in the mobile communication network, such as integrated Cellular and Ad hoc Relaying system (iCAR), Ad hoc GSM (A-GSM) and Opportunity Driven Multiple Access (ODMA).

3.3 WSN and Ad Hoc Network Market Outlook
WSN has broad application. However, it is generally not used as an independent system. This means the WSN market features "No Single Market, Market is Ubiquitous". According to relevant survey in USA, 2010 is expected to be the divide of the developing and mature periods of the WSN market. But Japan Ubiquitous Sensor Network Study Group predicts the developing period around 2007 and the mature period in 2010. However, all forecasts that the scale of the global WSN market in 2010 would exceed 100 billion US dollars. The present conclusion is that the market has been initiated and will grow fast, although there is a time difference between the prediction of the United States and Japan. The WSN industry has obtained the policy support of many governments, and numbers of market pushing forces are emerging. The WSN industrial chain has been set up. Relevant parts providers, software providers, system integrators and solution providers are pushing the industry from the R&D stage to the market.

    The Ad hoc network has been applied in diversified fields. Its application focuses on short-distance low-speed communication. Representative Ad hoc networks, including the RFID network, WPAN, mesh WiFi network, office network and home network, have been commercially used. It is forecasted that the Ad hoc network, as a complement to the fixed networks, will become an important part of the public communication network. It is possible that in the future it will have significant impact on the existing mobile communication. In addition, the impact can be expected on services and business operation models. In a large extent it will change the existing operation modes, as well as the mode of use of the telecom services.

4 Challenges
The leading nations in science and technology have given the development of the WSN and Ad hoc network supporting policies. The development of wireless technologies and increasing application demands are also pushing their development. However, there are still numbers of obstacles to the broad application of both networks.

    (1) Disunited Standards
    The release of relevant standards such as IEEE 802.15.4, ZigBee and IEEE 1451, has definitely fastened the development of WSN. However, there is no unified WSN standard yet. According to the study of On World Inc, most of available WSN products adopt their patented technologies only. For example, Dust persists in using its own technology; Ember keeps offering its EmberNet although it has set foot in the ZigBee field; and Zensys, one of initiators of the ZigBee Alliance, offers its own Z-Wave technology. The disunited standards bring about the problems of interoperability and usability of the WSN products. This makes part customers have a wait-and-see attitude toward the application of WSN, and holds back the growth of the WSN market.

    The study on the Ad hoc network has made great number of achievements. However, these achievements have their specific application environments. It is difficult for many problems to obtain universal answers, which is the reason that the protocols on Ad hoc have not been standardized yet. IETF has set up the Mobile Ad-hoc Networks (MANET) working group to focus on study of the unicast routing protocols for the Ad hoc network and their performance assessment. It has released several routing protocol drafts, and will further study the implementation of multicast routing and QoS. Furthermore, 3GPP and 3GPP2 have scheduled the Ad hoc network for next standardization study.

    (2) Immature Technologies
    Integrating the sensor, embedded computing, network and wireless communication, and distributed system, the WSN is facing a number of technical difficulties. Available technologies for network structure, topology control, communication, routing, security and data convergence cannot guarantee the large-scale application of the WSN. In addition, the cost and energy factors also limit the application.

    Similarly, there is no perfect technical solution for the Ad hoc network yet, although several research achievements have been made in MAC protocols, routing protocols, security, broadcast and multicast support, QoS, network management, and network interconnection.

    (3) Business Model
    WSN is a brand-new industry, while the operating model of the existing telecom services might be shaken by the Ad hoc network.  Therefore, effective business models play an extremely important role in the industrialization of both networks. They are worthwhile research subjects for equipment vendors, operators and industrial users.

5 Conclusions
Both WSN and the Ad hoc network are emerging technologies. They have potentials to strike numerous industries, and become an important part of our daily life. If the Internet is seen as human´s central nervous system, WSN is the nerve ending and sense organ, which are human´s major way to gather information. WSN closely connects with the physical world, therefore, it has become a basic method to obtain information in the information society. Ad hoc offers a new networking mode, which can effectively enlarge the network coverage and improve the efficiency of information transfer. Therefore, the two networks have great application potentials. Combined with the infrastructure networks, they are able to not only meet the demands of object markets, but also create numerous new market opportunities.

    It is obvious that WSN and the Ad hoc network are application-driven technologies. Therefore, their applications should be focused along with the study on the relevant essential technologies. Specific applications should be taken into consideration of technology selection. Besides, looking for technical breakthroughs, the standardization work should be enhanced to implement interconnection and interworking of equipment from different vendors as soon as possible. All these efforts are for accelerating their development. Moreover, innovative business models that play an extremely important role in pushing WSN and the Ad hoc markets forward should be the emphasized
research subjects.

References
[1] ON World Inc. Wireless Sensor Networks: Mass Market Opportunities [DB/OL]. http://www.onworld.com/html/wirelesssensorsrprt2.htm.
[2] Geng Zhicheng． Developments of Mobile Ad hoc Network [Z]． Communication Information Institute of China Academy of Telecommunications Research of MII, 2004, 5.
[3] Japan Ubiquitous Sensor Network Group．ユビキタスセンサネットワクの实现による社会的经济的效果 [DB/OL ]. http://www.soumu.go.jp/s-news/2004/pdf/040806_4_b2_6.pdf.

Manuscript received：2005-05-20